28 Professor Ernest Rutherford [Jan. 31, 



The law that governs the breaking up of atoms is very simple and 

 universal in its appKcation. For any simple substance, the average 

 number of atoms l^reaking up per second is proportional at any time 

 to the number present. In consequence, the amount of radio-active 

 matter decreases in a geometrical progression with the time. The 

 " period " of any radio-active product, i.e. the time for half the 

 matter to be transformed, is a definite and characteristic property of 

 the product which is uninfluenced by any of the laboratory agents at 

 our command. In fact, the period of any radio-active product, for 

 example, the radium emanation, if determined with sufficient accuracy, 

 might well be taken as a definite standard of time, independent of 

 all terrestrial influences. 



The law of radio-active transformation can be very simply and 

 aptly illustrated by an hydraulic analogy. Suppose we take a vertical 

 cylinder filled with water, with an opening near the base through 

 which the water escapes through a high resistance.* When the dis- 

 charge is started, the amount of water escaping per second is pro- 

 portional to the height of water above the zero level of the cylinder. 

 The height of water decreases in a geometrical progression with the 

 time in exactly the same way as the amount of radio-active matter 

 decreases. We can consequently take the height of the column of 

 water as representing the amount of radio-active matter A present at 

 any time. The quantity of water escaping per second is a measure 

 of the rate of disintegration of A and also of tlie amount of the new 

 substance B formed per second by the disintegration of A. The 

 " period " of the substance is controlled by the amount of resistance 

 in the discharge circuit. A high resistance gives a small flow of 

 water and a long period of transfoi'mation, and vice versa. By a 

 suitable arrangement we can readily trace out the decay curve for 

 such a case. A cork carrying a light vertical glass rod is floated on 

 the water in the cylinder. A light camel's hair brush is attached at 

 right angles, and moves over the surface of a smoked-glass plate. A 

 vertical line drawn on the glass through the point of contact of the 

 brush gives the axis of ordinates, while a horizontal line drawn 

 through the brush when the water has reached its lowest level gives 

 the axis of abscissae. If the glass plate is moved with uniform 

 velocity from the moment of starting the discharge, a curve is traced 

 on the glass which is identical in shape with the curve of decay of a 

 radio-active product, where the ordinates at any time represent the 

 relative amount of active matter present, and the abscissae time. 

 With such an apparatus we can illustrate in a simple way the increase 

 Avith time of radio-active matter B, which is supplied by the trans- 

 formation of a substance A. This will correspond, for example, to 

 the growth of the radium emanation with time in a quantity of 

 radium initially freed from emanation. Let us for convenience 



* A short glass tube in which is placed a plug of glass wool is very suitable. 



